Energy collector system having east-west extending linear reflectors

ABSTRACT

Disclosed herein are examples and variations of solar energy collector systems comprising an elevated linear receiver extending generally in an east-west direction, a polar reflector field located on the polar side of the receiver, and an equatorial reflector field located on the equatorial side of the receiver. Each reflector field comprises reflectors positioned in parallel rows which extend generally in the east-west direction. The reflectors in each field are arranged and positioned to reflect incident solar radiation to the receiver during diurnal east-west processing of the sun and pivotally driven to maintain reflection of the incident solar radiation to the receiver during cyclic diurnal north-south processing of the sun. Inter-row spacings of the reflectors on opposite sides of the receiver may be asymmetrical.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to Australian Provisional Patent Application 2006904628, filed 25 Aug. 2006, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to a solar energy collector system having linear reflectors, and in which relatively close spacing of at least some of the reflectors is facilitated by the positioning of all of the reflectors such that they extend generally in an east-west direction.

BACKGROUND OF THE INVENTION

Solar energy collector systems of the type referred to as Linear Fresnel Reflector (“LFR”) systems are relatively well known and are constituted by a field of linear reflectors that are arrayed in parallel rows and are orientated to reflect incident solar radiation to a common elevated receiver. The receiver is illuminated by the reflected radiation, for energy exchange, and the receiver typically extends parallel to the rows of reflectors. Also, the receiver normally (but not necessarily) is positioned between two adjacent fields of reflectors; and n spaced-apart receivers may be illuminated by reflections from (n+1) or, alternatively, (n−1) reflector fields, in some circumstances with any one receiver being illuminated by reflected radiation from two adjacent reflector fields.

In most known LFR system implementations the respective rows of reflectors are typically positioned to extend linearly in a north-south direction and the reflectors are pivotally mounted and driven to track east-west procession (i.e., apparent movement) of the sun during successive diurnal periods. This requires that adjacent rows of reflectors be spaced-apart by a predetermined distance, depending upon their distance from the associated receivers, in order to avoid shading or blocking of one reflector by another and, thus, in order to optimise reflection of incident radiation. This limits ground utilization to approximately 70% and diminishes system performance due to exacerbated spillage at the receiver of radiation from distant reflectors. As an alternative approach, a 1979 project design study (Ref Di Canio et al; Final Report 1977-79 DOE/ET/20426-1) proposed an east-west-extending LFR system.

SUMMARY OF THE INVENTION

The present invention provides in some variations a solar energy collector system comprising an elevated receiver and ground level reflector fields located generally to the equatorial and polar sides of the receiver, with each reflector field comprising reflectors which are positioned linearly in parallel rows which extend generally in an east-west direction. The reflectors in both fields are arranged and positioned to reflect incident solar radiation to the receiver during diurnal east-west processing of the sun, and are pivotally driven to maintain reflection of the incident solar radiation to the receiver during cyclic diurnal north-south processing of the sun.

In referring above to the reflector rows extending “generally” in an east-west direction, it is meant that the reflector rows lie orthogonal to the earth's magnetic axis within a tolerance of ±45°. Similarly, in referring to the reflector fields being located “generally” to the equatorial and polar sides of the receiver, it is meant that the reflector fields have a common axis that aligns with the earth's magnetic axis within a tolerance of ±45°.

With the above defined arrangement of, and drive applied to, the reflectors, the reflectors to the equatorial side of the receiver will, during most of the year, be disposed at an angle to the horizontal that is substantially more acute (i.e., smaller) than that of the reflectors that are positioned at the polar side of the receiver. Also, the inter-reflector spacing provided at the equatorial side may be smaller than at the polar side. This leads to improved ground coverage and reflectors that are, on average, closer to the receiver, this in turn providing for a smaller image and less spillage. Thus, the invention provides for (north-south) asymmetrical inter-reflector spacing and, consequentially, for efficient solar collection and enhanced ground coverage. A reflector-to-ground area ratio of almost 80% may be achieved without the occurrence of serious shading and blockage, as compared with the 70% figure applicable to prior art systems.

The receiver may have a substantially horizontal aperture. In some variations, the receiver has a substantially horizontal aperture closed with a cover that is substantially transparent to solar radiation. This arrangement provides for convection suppression with the air behind the cover being maintained in a stagnant state. Stagnant air is an excellent insulator and the described arrangement provides for efficient performance without there being a need for expensive vacuum-insulated absorbers.

As a further optional aspect of the invention, two or more receivers may be sited within a single energy collection system comprising multiple reflector fields.

The receiver(s) may take any desired form, depending upon the nature of energy exchange required from the reflected solar radiation. In the case of a requirement for heat energy exchange between the reflected solar radiation and a working fluid such as water, the receiver might typically comprise a system as disclosed in International Patent Application PCT/AU2005/000208, filed 17 Feb. 2005 by the present Applicant. The reflectors also may take any desired form but might typically comprise units as disclosed in International Patent Applications PCT/AU2004/000883 and PCT/AU2004/000884, filed 1 Jul. 2004 by the present Applicant.

The invention will be more fully understood from the following description of illustrative examples of solar energy collector systems, the description being provided with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in a schematic way a prior art solar collector system having a single receiver and reflector fields located to the east and west of the receiver.

FIG. 2 shows, again in a schematic way, a portion of a Linear Fresnel Reflector (“LFR”) solar collector system in accordance with one variation of the present invention, the system having a single receiver and reflector fields located to the north and south of the receiver.

FIG. 3 shows a schematic representation of the LFR system of FIG. 2 as viewed in the direction of Arrow 3 as shown in FIG. 2.

FIG. 4 shows a more detailed representation of an example LFR system of the type shown in FIGS. 2 and 3 but with two receivers.

FIGS. 5A-5C show schematic representations of an example receiver structure, with FIG. 5C showing a portion of the receiver structure which is encircled by circle A in FIG. 5B.

FIGS. 6A-6D show example fluid flow arrangements through a receiver.

FIG. 7 shows a perspective view of a reflector according to one variation.

FIG. 8 shows on an enlarged scale a portion of a mounting arrangement for a reflector.

FIG. 9 shows on an enlarged scale a portion of a reflector and a drive system for the reflector according to one variation.

DETAILED DESCRIPTION OF THE INVENTION AND PRIOR ART

Reference is made firstly to the prior art LFR (solar collector) system as shown schematically in FIG. 1. This system comprises two fields 10 and 11 of ground-mounted linear reflectors 12 that extend linearly in parallel rows and are orientated to reflect incident solar radiation to a common (i.e., shared) elevated receiver 13. The receiver is illuminated by the reflected solar radiation to effect energy exchange with, for example, fluid that is channelled through the receiver, and the receiver 13 extends parallel to the rows of reflectors 12.

The rows of reflectors 12 in both of the fields 10 and 11 extend linearly in a north-south direction and all of the reflectors are pivotally mounted and are driven through an angle approaching 90° whilst tracking east-west procession of the sun (as indicated by the direction of arrow 14) during successive diurnal periods. The two receiver fields 10 and 11 are symmetrically disposed to the east and west of the receiver 13, and adjacent rows of the reflectors 12 are spaced-apart by a predetermined distance, depending upon their distance from the receiver 13, in order to avoid shading of one reflector by another and consequential blockage of either incident or reflected radiation. As indicated previously, this spacing requirement of the prior art system limits ground utilization and diminishes system performance due to exacerbated spillage at the receiver of radiation from distant reflectors.

An example LFR system in accordance with the present invention, shown schematically in FIGS. 2 and 3, comprises an elevated receiver 15 that is positioned between two ground level reflector fields 16 and 17. One of the fields 16 is located to the equatorial side of the receiver (i.e., to the southern side S in the case of a northern hemisphere system) and the other field 17 is located to the polar side of the receiver 15 (i.e., to the northern side N in the case of a northern hemisphere system). The respective reflector fields 16 and 17 comprise reflectors 18 a and 19 a which are positioned linearly in parallel rows and, in contrast with the prior art system as illustrated in FIG. 1, the rows of reflectors 18 a and 19 a extend generally in an east-west direction.

The reflectors 18 a and 19 a are arranged and positioned to reflect incident solar radiation to the receiver 15 during diurnal east-west processing of the sun in the direction indicated by arrow 20 (FIG. 3). Additionally, the reflectors 18 a and 19 a are pivotally driven to maintain reflection of the incident solar radiation to the receiver 15 during cyclic diurnal north-south processing of the sun in the (inclining and declining) directions indicated by arrow 21 (FIG. 2). However, having regard to the fact that the diurnal sun processes through an angle less than 90° in the north-south direction, as compared with an angle approaching 180° in the east-west direction, the total pivotal movement imparted to the reflectors 18 a and 19 a is less than 45° during each diurnal period. Consequently, in the case of the embodiment of the system that is illustrated in FIGS. 2 and 3, the reflectors 18 a (i.e., those located to the equatorial side of the receiver) will at all times be disposed at an angle to the horizontal that is substantially more acute than that of the reflectors 19 a that are positioned at the polar side of the receiver. Hence, the potential for shading of reflectors at the equatorial side of the receiver will be small relative to that applicable to the reflectors at the polar side. This, as previously mentioned, permits close spacing of the equatorial side reflector rows and, thus, results in a reduction in the total field area relative to that required for the arrangement illustrated in FIG. 1. Also, because of the close-to-horizontal disposition of the reflectors 18 a at the equatorial side of the receiver, the resultant close-packed reflectors can be located closer to the receiver than in the case of the prior art system illustrated in FIG. 1, thus decreasing image size and reducing radiation spillage.

As shown in more detail in FIG. 4, another example LFR system of the type shown in FIGS. 2 and 3 comprises notionally separate fields 16 and 17 of ground mounted reflectors 18 a and 19 a that are aligned (and, e.g., interconnected) in parallel rows 18, 19 that extend generally in the east-west direction. In addition, this example LFR system comprises two parallel receivers 15, each of which is constituted by aligned (and, e.g., interconnected) receiver structures 15 a. A complete LFR system might occupy a ground area within the range 5×10 m² to 25×10⁶ m² and the complete system as illustrated in FIG. 4 may be considered as a portion only of a larger LFR system.

The reflectors 18 a and 19 a may be driven collectively or regionally, as rows or individually, to track procession of the sun and they are orientated to reflect incident radiation to respective ones of the receivers 15, in the manner described previously with reference to FIGS. 2 and 3.

In the example system illustrated in FIG. 4, each receiver 15 receives reflected radiation from twelve rows of reflectors 18 a and 19 a. Thus, each receiver 15 is illuminated by reflected radiation from six rows of reflectors 18 a at one side of the receiver and from six rows of reflectors 19 a at the other side. Each row of the reflectors 18 a and 19 a and, hence, each receiver 15 might typically have an overall length of 300 to 600 metres, and the parallel, east-west extending receivers 15 might typically be spaced apart by 30 to 35 metres. The receivers 15 are supported, for example, at a height of approximately 11 metres by stanchions 22 which are stayed by ground-anchored guy wires 23. In the example of FIG. 4, each of the receivers 15 comprises a plurality of receiver structure 15 a that are connected together co-linearly. The receiver structures 15 a might have, for example, a length of the order of 12 metres and an overall width of the order of 1.4 meters.

The receivers 15 may be, for example, of the type described in the previously mentioned International Patent Application numbered PCT/AU2005/000208, and the disclosure of that Patent Application is incorporated herein by reference.

Referring to FIGS. 5A-5C, in some variations each receiver structure 15 a comprises an inverted trough 24 which might typically be formed from stainless steel sheeting and which, as best seen in FIG. 5B, has a longitudinally extending channel portion 26 and flared side walls 27 that, at their margins, define an aperture of the inverted trough through which solar radiation incident from the reflectors may enter the trough. In the illustrated variation, the trough 24 is supported and provided with structural integrity by side rails 28 and transverse bridging members 29, and the trough is surmounted by a corrugated steel roof 30 that is carried by arched structural members 31.

In the illustrated variation, the void between the trough 24 and the roof 30 is filled with a thermal insulating material 32, typically a glass wool material, and desirably with an insulating material that is clad with a reflective metal layer. The function of the insulating material and the reflective metal layer is to inhibit upward conduction and radiation of heat from within the trough.

A longitudinally extending window 25 is provided to interconnect the side walls 27 of the trough. The window is formed from a sheet of material that is substantially transparent to solar radiation and it functions to define a closed (heat retaining) longitudinally extending cavity 33 within the trough. Window 25 may be formed from glass, for example.

In the receiver structure as illustrated, longitudinally extending (e.g., stainless steel) absorber tubes 34 are provided for carrying heat exchange fluid (typically water or, following heat absorption, water-steam or steam). The actual number of absorber tubes may be varied to suit specific system requirements, provided that each absorber tube has a diameter that is small relative to the dimension of the trough aperture between the side walls 28 of the trough, and the receiver structure might typically have between six and thirty absorber tubes 34 supported side-by side within the trough.

The actual ratio of the absorber tube diameter to the trough aperture dimension may be varied to meet system requirements but, in order to indicate an order of magnitude of the ratio, it might typically be within the range 0.01:1.00 to 0.10:1.00. Each absorber tube 34 might have an outside diameter, for example, of 33 mm. With an aperture dimension of, for example, 1100 mm, the ratio of the absorber tube diameter to the aperture dimension would be 0.03:1.00.

With the above described arrangement the plurality of absorber tubes 34 will effectively simulate a flat plate absorber, as compared with a single-tube collector in a concentrating trough. This provides for increased operating efficiency, in terms of a reduced level of heat emission from the upper, non-illuminated circumferential portion of the absorber tubes. Moreover, by positioning the absorber tubes in the inverted trough in the manner described, the underside portion only of each of the absorber tubes is illuminated with incident radiation, this providing for efficient heat absorption in absorber tubes that carry steam above water.

In the illustrated variation, the absorber tubes 34 are freely supported by a series of parallel support tubes 35 which extend orthogonally between side walls 36 of the channel portion 26 of the inverted trough, and the support tubes 35 may be carried for rotational movement by spigots 37. This arrangement accommodates expansion of the absorber tubes and relative expansion of the individual tubes. Disk-shaped spacers 38 are carried by the support tubes 35 and serve to maintain the absorber tubes 34 in spaced relationship.

Each of the absorber tubes 34 may be coated with a solar absorptive coating. The coating may comprise, for example, a solar spectrally selective surface coating that remains stable under high temperature conditions in ambient air or, for example, a black paint that is stable in air under high-temperature conditions.

In some variations fluid flow through absorber tubes 34 may be in parallel unidirectional streams. Other flow arrangements may also be used, however. FIG. 6A of the drawings shows diagrammatically one example flow control arrangement for controlling flow of heat exchange fluid into and through four in-line receiver structures 15 a of a receiver. As illustrated, each of the fluid lines 34A, B, C and D is representative of four of the absorber tubes 34 as shown in FIGS. 5A-5C.

Under the controlled condition illustrated in FIG. 6A, in-flowing heat exchange fluid is first directed along forward line 34A, along return line 34B, along forward line 34C and finally along and from return line 34D. This results in fluid at a lower temperature being directed through tubes that are located along the margins of the inverted trough and a consequential emission reduction when radiation is concentrated over the central region of the inverted trough. An electrically actuated control device 39 may be provided to enable selective control over the channelling of the heat exchange fluid in some variations.

Alternative fluid flow conditions may be established to meet load demands and/or prevailing ambient conditions, and provision may effectively be made for a variable aperture receiver structure by closing selected ones of the absorber tubes. Thus, variation of the effective absorption aperture of each receiver structure and, hence, of a complete receiver may be achieved by controlling the channelling of the heat exchange fluid in the alternative manners shown in FIGS. 6B to 6D.

Referring again to FIGS. 4 and 5A-5C, in some variations in which the aperture of the inverted trough 24 and, hence, the window 25 are positioned with a substantially horizontal disposition it may be advantageous that the reflector fields 16 and 17 be formed with substantially equidistant extents at each side of the receiver 15 in order to avoid surface reflection from the window 25. Where individual reflectors are large relative to the receiver, the optimum arrangement may have an equal number of reflectors on either side of the receiver, but where the individual reflectors are small relative to the receiver it may be possible to pack more of them on the equatorial side of the receiver than on the polar side, so that some reflector field ground area difference may occur.

The reflectors 18 a and 19 a may be, for example, of the type described in the previously mentioned International Patent Applications numbered PCT/AU2004/000883 and PCT/AU2004/000884, and the disclosures of those Patent Applications are incorporated herein by reference.

Referring to FIG. 7, in some variations a reflector (e.g., 18 a, 19 a) comprises a carrier structure 40 to which a reflector element 41 is mounted. The carrier structure itself comprises an elongated panel-like platform 42 which is supported by a skeletal frame structure 43. The frame structure includes two hoop-like end members 44.

The members 44 are cantered on and extend about an axis of rotation that is approximately coincident with a central, longitudinally-extending axis of the reflector element 41. The axis of rotation does not need to be exactly coincident with the longitudinal axis of the reflector element but the two axes desirably are at least adjacent one another.

In terms of overall dimensions of the reflector, the platform 42 is, for example, approximately twelve meters long and the end members 14 are approximately two meters in diameter.

The platform 42 comprises a corrugated metal panel and the reflector element 41 is supported upon the crests of the corrugations. The corrugations extend parallel to the direction of the longitudinal axis of the reflector element 41, and the platform 42 is carried by, for example, six transverse frame members 45 of the skeletal frame structure 43. End ones of the transverse frame members 45 effectively comprise diametral members of the hoop-like end members 44.

The transverse frame members 45 comprise rectangular hollow section steel members and each of them is formed with a curve so that, when the platform 42 is secured to the frame members 45, the platform is caused to curve concavely (as viewed from above in FIG. 7) in a direction orthogonal to the longitudinal axis of the reflector element 41. The same curvature is imparted to the reflector element 41 when it is secured to the platform 42.

The radius of curvature of the transverse frame members 45 is, for example, in the range of twenty to fifty meters and preferably of the order of thirty-eight meters.

The skeletal frame 43 of the carrier structure 40 also comprises a rectangular hollow section steel spine member 46 which interconnects the end members 44, and a space frame which is fabricated from tubular steel struts 47 connects opposite end regions of each of the transverse frame members 45 to the spine member 46. This skeletal frame arrangement, together with the corrugated structure of the platform 42 provides the composite carrier structure 41 with a high degree of torsional stiffness.

The hoop-like end members 44 are formed from channel section steel, for example, such that each end member is provided with a U-shaped circumferential portion and, as shown in FIG. 8, each of the members 44 is supported for rotation on a mounting arrangement that comprises two spaced-apart rollers 48. The rollers 48 are positioned to track within the channel section of the respective end members 44, and the rollers 48 provide for turning (i.e., rotation) of the carrier structure 40 about the axis of rotation that is approximately coincident with the longitudinal axis of the reflector element 41.

As also shown in FIG. 8, a hold-down roller 48 a is located adjacent the support rollers 48 and is positioned within the associated end member 44 to prevent lifting of the reflector under adverse weather conditions.

A drive system, as shown in FIG. 9, is provided for imparting drive to the carrier structure 40 and, hence, to the reflector element 41. The drive system comprises, for example, an electric motor 49 having an output shaft coupled to a sprocket 50 by way of reduction gearing 51. The sprocket 50 meshes with a link chain 52 through which drive is directed to the carrier structure 40.

The link chain 52 extends around and is fixed to the periphery of the outer wall 53 of the channel-section of one of the end member 44. That is, the link chain 52 affixed to the end member effectively forms a type of gear wheel with which the sprocket 50 engages.

With, for example, the end member 44 having a diameter in the order of 2.0 m and the sprocket 50 having a pitch circle diameter of 0.05 m, reduction gearing and torque amplification in the order of (40.r):1 may be obtained, where r is the reduction obtained through gearing at the output of the electric motor 49.

The reflector element 41 is formed, for example, by butting together five glass mirrors, each of which has the dimensions, for example, of 1.8 m×2.4 m. A silicone sealant may be employed to seal gaps around and between the mirrors and to minimize the possibility for atmospheric damage to the rear silvered faces of the mirrors. The mirrors may be secured to the crests of the platform 12 by a urethane adhesive, for example.

In some variations, the mirrors have a thickness of 0.003 m and, thus, they may readily be curved in situ to match the curvature of the supporting platform 42.

Depending upon requirements, two or more of the above described reflectors may be positioned linearly in a row and be connected one to another by way of adjacent ones of the hoop-like end members 44. In such an arrangement a single drive system may be employed for imparting drive to multiple reflectors.

This disclosure is illustrative and not limiting. Further modifications will be apparent to one skilled in the art in light of this disclosure and are intended to fall within the scope of the appended claims. 

1. A solar energy collector system comprising: an elevated linear receiver extending generally in an east-west direction; a polar reflector field located on the polar side of the receiver; and an equatorial reflector field located on the equatorial side of the receiver; wherein each reflector field comprises reflectors positioned in parallel side-by-side rows which extend generally in the east-west direction, the reflectors in each field are arranged and positioned to reflect incident solar radiation to the receiver during diurnal east-west processing of the sun and pivotally driven to maintain reflection of the incident solar radiation to the receiver during cyclic diurnal north-south processing of the sun, and inter-row spacings of the reflectors on opposite sides of the receiver are asymmetrical.
 2. The solar energy collector system of claim 1 wherein inter-row spacings in the equatorial reflector field are smaller than corresponding inter-row spacings in the polar reflector field.
 3. The solar energy collector system of claim 1, wherein reflectors in the equatorial reflector field are located closer to the receiver than are corresponding reflectors in the polar reflector field.
 4. The solar energy collector system of claim 1, wherein the reflector to ground area ratio is greater than about 70%.
 5. The solar energy collector system of claim 1, wherein the reflector to ground area ratio is approximately 80%.
 6. The solar energy collector system of claim 1, wherein solar radiation reflected to the receiver heats a working or heat exchange fluid.
 7. The solar energy collector system of claim 6, wherein the working or heat exchange fluid comprises water.
 8. The solar energy collector system of claim 1, wherein the receiver comprises an inverted trough having a substantially horizontal aperture through which solar radiation reflected by the reflectors may pass.
 9. The solar energy collector system of claim 8, wherein the inverted trough comprises a plurality of absorber tubes that are arranged side by side, each absorber tube having a diameter small relative to the aperture of the trough and, in use, carrying a working or heat exchange fluid.
 10. The solar energy collector system of claim 9, wherein the ratio of each absorber tube diameter to the trough aperture is in the range of 0.01:1.00 to 0.10:1.00.
 11. The solar energy collector system of claim 9, wherein the trough comprises from about 10 to about 30 of the absorber tubes.
 12. The solar energy collector system of claim 9, wherein the working or heat exchange fluid comprises water.
 13. The solar energy collector system of claim 9, wherein the absorber tubes are configured such that, in use, working or heat exchange fluid flows through the absorber tubes in parallel streams in a common direction.
 14. The solar energy collector system of claim 9, wherein the absorber tubes are configured such that, in use, in-flowing working or heat exchange fluid is first directed through outer ones of the absorber tubes and then directed through inner ones of the absorber tubes.
 15. The solar energy collector system of claim 9, wherein the absorber tubes are freely supported by rotating cylindrical supports.
 16. The solar energy collector system of claim 8, wherein the aperture is closed with a cover that is substantially transparent to solar radiation.
 17. The solar energy collector system of claim 1, wherein a reflector has a radius of curvature of about 20 to about 50 meters.
 18. The solar energy collector system of claim 1, wherein a reflector comprises: a reflector element; and a carrier structure including a platform that supports the reflector element and hoop-like end members between which the platform extends, the hoop-like end members having a diameter greater than a width of the reflector element and accommodating rotation of the carrier structure about an axis of rotation that is substantially coincident with a longitudinal axis of the reflector element.
 19. The solar energy collector system of claim 18, wherein the reflector element has a radius of curvature of about 20 to about 50 meters.
 20. The solar energy collector system of claim 18, wherein the reflector element is secured to the platform in a manner such that a curvature of the platform is imparted to the reflector element.
 21. The solar energy collector system of claim 1, wherein the receiver has a length of about 300 to about 600 meters.
 22. The solar energy collector system of claim 1, wherein the linear receiver is one of a plurality of linear receivers arranged side-by-side and spaced apart by about 30 to about 35 meters.
 23. The solar energy collector system of claim 1, wherein a row of reflectors has a length of about 300 to about 600 meters.
 24. The solar energy collector system of claim 1, wherein the reflector fields are located at ground level.
 25. The solar energy collector system of claim 1, wherein the equatorial and polar reflector fields include the same number of rows of reflectors.
 26. The solar energy collector system of claim 1, wherein the equatorial reflector field includes more rows of reflectors than does the polar reflector field.
 27. A method of making a solar energy system comprising: positioning and arranging rows of reflectors on a polar side of an elevated generally east-west extending linear receiver to reflect solar radiation to the receiver; and positioning and arranging rows of reflectors on an equatorial side of the receiver to reflect solar radiation to the receiver with spacings between rows on the equatorial side of the receiver smaller than spacings between corresponding rows on the polar side of the receiver.
 28. The method of claim 27, further comprising positioning rows on the equatorial side of the receiver closer to the receiver than corresponding rows on the polar side of the receiver.
 29. The method of claim 27, further comprising configuring the reflectors to be pivotally driven to maintain reflection of solar radiation to the receiver during diurnal north-south processing of the sun.
 30. The method of claim 27, further comprising arranging a plurality of absorber tubes side-by-side in an inverted trough in the receiver to absorb light reflected by the reflectors and thereby heat a working or heat exchange fluid that, in use, flows through the absorber tubes, the trough having an aperture through which solar radiation reflected by the reflectors may pass to be incident on the absorber tubes.
 31. The method of claim 30, wherein the aperture is substantially horizontal.
 32. The method of claim 30, wherein the absorber tubes have a diameter that is small relative to the aperture of the trough.
 33. The method of claim 32, wherein the ratio of the absorber tube diameter to the trough aperture is in the range of 0.01:1 to 0.10:1.
 34. The method of claim 30, wherein the trough comprises about 10 to about 30 of the absorber tubes arranged side-by-side.
 35. The method of claim 30, further comprising closing the aperture with a cover that is substantially transparent to solar radiation.
 36. The method of claim 30, further comprising configuring the absorber tubes such that, in use, working or heat exchange fluid flows through the absorber tubes in parallel streams in a common direction.
 37. The method of claim 30, further comprising configuring the absorber tubes such that, in use, in-flowing working or heat exchange fluid is first directed through outer ones of the absorber tubes and then directed through inner ones of the absorber tubes.
 38. The method of claim 27, wherein a reflector has a radius of curvature of about 20 to about 50 meters.
 39. The method of claim 27, wherein the receiver has a length of about 300 to about 600 meters.
 40. The method of claim 27, wherein a row of reflectors has a length of about 300 to about 600 meters. 